hw/dma/xlnx_csu_dma: don't throw guest errors when stopping the SRC DMA

hw/dma/xlnx_csu_dma: don't throw guest errors when stopping the SRC DMA

UG1087 states for the source channel that: if SIZE is programmed to 0, and the
DMA is started, the interrupts DONE and MEM_DONE will be asserted.

This implies that it is allowed for the guest to stop the source DMA by writing
a size of 0 to the SIZE register, so remove the LOG_GUEST_ERROR in that case.

While at it remove the comment marking the SIZE register as write-only.

See: https://docs.xilinx.com/r/en-US/ug1087-zynq-ultrascale-registers/CSUDMA_SRC_SIZE-CSUDMA-Register

Signed-off-by: Frederic Konrad <fkonrad@amd.com>
Reviewed-by: Francisco Iglesias <francisco.iglesias@amd.com>
Message-id: 20231124143505.1493184-4-fkonrad@amd.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>